Drying using inert gas from the shore
Drying may be carried out as part of the inerting procedure when taking inert gas from the shore (see 7.2) and this is now commonly done. This method has the advantage of providing the dual functions of lowering the moisture content in tank atmospheres to the required dew point and, at the same time, lowering the oxygen content. A disadvantage of this and the following method is that more inert gas is used than if it is simply a question of reducing the oxygen content to a particular value.
Drying using inert gas from ship's plant
Drying can also be accomplished at the same time as the inerting operation when using the ship's inert gas generator but satisfactory water vapour removal is dependent on the specification of the inert gas system. Here, the generator must be of suitable capacity and the inert gas of suitable quality — but the necessary specifications are not always a design feature of this equipment. The ship's inert gas generator is sometimes provided with both a refrigerated dryer and an adsorption drier (see 4.7) which, taken together, can reduce dew points at atmospheric pressure to -45°C or below.
On board air-drying systems
An alternative to drying with inert gas is by means of an air-drier fitted on board. The principle of operation is shown in Figure 7.1. In this method, air is drawn from
Figure 7.1 Air Drying — operational cycle
the cargo tank by a compressor or provided by the on board inert gas blower (without combustion) and passed through a refrigerated drier. The drier is normally cooled by R22 refrigerant. Here the air is cooled and the water vapour is condensed out and drained off. The air leaving the drier is, therefore, saturated at a lower dew point. Further reduction of the dew point can be achieved by a silica gel after-drier fitted downstream. Thereafter, the air may be warmed back to ambient conditions by means of an air heater and returned to the cargo tank. This process is continued for all ship tanks (and pipelines) until the dew point of the in-tank atmosphere is appropriate to carriage conditions.
7.2.3 Inerting — before loading
Inerting cargo tanks, cargo machinery and pipelines is undertaken primarily to ensure a non-flammable condition during subsequent gassing-up with cargo. For this purpose, oxygen concentration must be reduced from 21 per cent to a maximum of five per cent by volume although lower values are often preferred — see Table 2.3(b).
However, another reason for inerting is that for some of the more reactive chemical gases, such as vinyl chloride or butadiene, levels of oxygen as low as 0.1 per cent may be required to avoid a chemical reaction with the incoming vapour. Such low oxygen levels can usually only be achieved by nitrogen inerting provided from the shore (see 2.5 and 4.7.3).
|
|
|
There are two procedures which can be used for inerting cargo tanks: displacement or dilution. These procedures are discussed below.
Inerting by displacement
Inerting by displacement, also known as piston purge, relies on stratification of the cargo tank atmosphere based on the difference in vapour densities between the gas
Figure 7.2 Inerting cargo tanks by the displacement method
entering the tank and the gas already in the tank. The heavier gas (see Table 2.5) is introduced beneath the lighter gas at a low velocity to minimise turbulence. If good stratification can be achieved, with little mixing at the interface, then just one tank volume of the incoming inert gas is sufficient to change the atmosphere. In practice mixing occurs and it is necessary to use more than one tank-volume of inert gas. This amount may vary by up to four times the tank volume, depending on the relative densities of the gases together with tank and pipeline configurations. There is little density difference between air and inert gas (see Table 2.4); inert gas from a combustion generator is slightly heavier than air while nitrogen is slightly lighter. These small density differences make inerting by displacement difficult to achieve and usually the process becomes part displacement and part dilution (discussed below). Combustion-generated inert gas is usually introduced through the liquid loading line with the effluent being exhausted through the vapour line into the vent header. Figure 7.2 shows the inerting of a cargo tank by the displacement method. The symbols used in this and the cargo handling diagrams which follow are identified at the beginning of this book.
Inerting by displacement is an economical procedure as it uses the least amount of inert gas and takes the shortest time. However, it is only practical when mixing with the initial tank vapour can be limited. If the tank shape and the position pipe-entries are suitable for the displacement method, then results will be improved by inerting more than one tank at a time. This should be done with the tanks aligned in parallel. The sharing of the inert gas generator output between tanks reduces gas inlet speeds, so limiting vapour mixing at the interface. At the same time the total inert gas flow increases due to the lower overall flow resistance. Tanks being inerted in this way should be monitored to ensure equal sharing of the inert gas flow.
|
|
|
Inerting by dilution
When inerting a tank by the dilution method, the incoming inert gas mixes, through turbulence, with the gas already in the tank. The dilution method can be carried out in several different ways and these are described below:—
Дата добавления: 2018-02-28; просмотров: 520; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!